Continuous execution of begin statements does not behave as expected

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

prepare:

```sql
create table t1 (id int primary key, name varchar(10));
insert into t1 values (1, 'abc');
set @@global.tidb_txn_mode='optimistic';
set @@global.tidb_disable_txn_auto_retry=0;
```

| Session-A | Session-B |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------|
| begin optimistic; | |
| begin optimistic; | |
| update t1 set name = 'session 1' where id = 1 and name = 'abc'; | |
| | begin optimistic; |
| | update t1 set name = 'session 2' where id = 1 and name = 'abc'; |
| | commit; |
| commit; (9007, 'Write conflict, txnStartTS=446466768598466563, conflictStartTS=446466770507661313, conflictCommitTS=446466771718766593, key={tableID=49, handle=1} primary={tableID=49, handle=1} [try again later]') | |

Session-A commit failed.

### 2. What did you expect to see? (Required)

Session-A commit success. Since it is a optimistic transaction and retryable.

### 3. What did you see instead (Required)

Session-A commit failed.

### 4. What is your TiDB version? (Required)

v4.0.0

### Investigate

The main reason is when continuous execution of `begin optimistic` statement, will `NewTxn() inside a transaction auto commit`, then it doesn't set the `CouldRetry` info, then the transaction become not retryable.

https://github.com/pingcap/tidb/blob/838b6a0cf2df2d1907508e56d9de9ba7fab502e5/session/session.go#L1428-L1433